The present invention relates to optical communications, and more particularly to optical amplification systems and methods.
In fiber optic transmission systems, optical communications signals are typically transmitted in two or more discrete communications bands on a common optical fiber. Each of the communications bands consists of optical signals in a particular corresponding wavelength range, and includes a plurality of closely spaced channels at successive wavelength intervals within the wavelength range of the band. Due to the long transmission distances that the optical signals typically travel through the optical fiber medium, it is often necessary to amplify the optical signals during the course of their transmission, to compensate for signal strength losses occurring in the fiber.
A typical optical amplifier, such as an erbium-doped fiber amplifier for example, is generally able to amplify only a limited bandwidth range, such as one communications band for example, and is generally not capable of simultaneously amplifying all of the communications bands. Accordingly, one approach to optical signal amplification involves the use of interface filters to separate the communications bands. Each communications band is then directed through a separate respective optical amplifier, having input and output power specifications corresponding to the expected signal strength loss and required signal strength output for that particular communications band. The amplified communications bands may then be passed through a second set of interface filters, to recombine the communications bands onto a common fiber.
However, this technique tends to suffer from a difficulty known as inter-band cross-talk, which results from inherent properties of the interface filters. In practice, filters are non-ideal. Thus, a filter designed to transmit only a desired communications band cannot perfectly block all wavelengths outside the band, but rather, will tend to block such out-of-band wavelengths in proportion to their separation away from the desired band. Thus, wavelengths just outside the desired band may still be transmitted by the filter at significant signal strengths, whereas wavelengths further away from the wavelength range of the desired band will be increasingly blocked, in proportion to their wavelength separation or distance from the boundaries of the desired band. As a result, in the case of adjacent optical communications bands, wavelengths of a first communications band that are close to an adjacent second communications band may travel not only along the amplification path for the first band, but may also be inadvertently passed along the amplification path for the second band. When these latter wavelengths from the second amplification path are recombined with signals at similar wavelengths from the first amplification path, coherent interference may result, severely degrading system performance.
In order to reduce inter-band cross-talk to acceptable levels, xe2x80x9cdeadbandsxe2x80x9d, or unused wavelength ranges, are typically provided between adjacent communications bands, to increase the spacing between bands, so that for a given communications band, the nearest wavelengths from adjacent bands will be sufficiently blocked by the interface filters. Unfortunately, these deadbands amount to a waste of significant potential communications bandwidth.
In addition, today""s increasing demand for bandwidth is tending to result in demand for wider communications bands (as well as narrower channel spacing within a given band). However, a wider communications band requires a wider corresponding transmission window of the filter, which, as a matter of filter design, tends to require a shallower xe2x80x9cslopexe2x80x9d of the filter response. This results in undesired transmission of out-of-band signals at appreciable signal strengths over an even wider range of out-of-band transmission wavelengths, which increases the effects of inter-band cross-talk. Thus, in order to widen communications bands to accommodate additional bandwidth, it would be necessary to increase the width of the deadbands between the communications bands to prevent unacceptable cross-talk effects, thereby resulting in even greater amounts of wasted potential communications bandwidth.
Although it may be possible to reduce cross-talk effects by adding further interface filters either before or amplification, these approaches are not desirable, as the increased insertion loss resulting from additional pre-amplification filtering tends to result in greater noise in the system, whereas additional post-amplification filtering tends to result in inefficient output power loss, which either partially defeats the purpose of signal amplification or requires greater amplification power to compensate.
Conversely, although at least one approach to reducing cross-talk involves use of a filter in a mid-stage of an amplification path to filter out an entire adjacent communications band, the width of such a filter and its correspondingly shallow-sloped out-of-band response would tend to inadvertently filter at least some signals in the desired communications band, unless significant deadbands were provided between adjacent communications bands.
Accordingly, there is a need for an improved optical amplification method.
The present invention addresses the above need by providing, in accordance with one aspect of the invention, an optical amplification system including an optical interface filter and an optical amplifier. The optical interface filter is configured to separate a desired communications band from at least one other communications band. The optical amplifier is in communication with the interface filter for amplifying the desired communications band, and includes at least one suppression filter configured to attenuate a wavelength subset of the at least one other communications band nearest the desired communications band and narrower than the at least one other communications band.
Thus, as the suppression filter is configured to attenuate a wavelength subset of the at least one other communications band narrower than the other communications band, the suppression filter has a narrower attenuation range and therefore tends to have a more steeply-sloped transition from attenuated wavelengths to transmitted wavelengths than wider-range filters designed to filter entire bands. Therefore, the deadband or deadbands between the desired communications band and the at least one other communications band may be significantly reduced in width compared to existing systems, without any appreciable inadvertent attenuation by the suppression filter of the desired communications band.
At the same time, it will be appreciated that the inadequacy of the initial filtration provided by the interface filter, resulting from the relatively shallowsloped transition of the interface filter from transmitted to filtered wavelengths, is most appreciable at the region of the other communications band nearest the desired communications band, and is not appreciable far away from the desired communications band. Therefore, as the suppression filter is configured to attenuate the wavelength subset of the at least one other communications band nearest the desired communications band, the attenuation wavelength range of the suppression filter may still be broad enough to attenuate any wavelengths of the at least one other communications band having appreciable signal strengths, thereby suppressing inter-band cross-talk.
Thus, such a system permits deadbands to be significantly reduced in width, thereby reducing wasted potential communications bandwidth, while at the same time adequately suppressing the increased inter-band cross-talk that would otherwise result from a reduction of deadband width.
The suppression filter is preferably configured to attenuate the wavelength subset while forwarding other wavelengths with negligible attenuation. If desired, the suppression filter may be configured to attenuate, as the subset, a wavelength range an order of magnitude narrower than the at least one other communications band.
In accordance with another aspect of the invention, there is provided an optical amplification method, involving separating a desired communications band from at least one other communications band, and amplifying the desired communications band while attenuating a wavelength subset of the at least one other communications band nearest the desired communications band and narrower than the at least one other communications band.
In accordance with another aspect of the invention, there is provided an optical amplification system. The system includes provisions for separating a desired communications band from at least one other communications band, and further includes provisions for amplifying the desired communications band. The provisions for amplifying include provisions for attenuating a wavelength subset of the at least one other communications band nearest the desired communications band and narrower than the at least one other communications band.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.